Photosensitive matrix electronic sensor

ABSTRACT

A photosensitive matrix sensor include a faceplate of caesium iodide CsI mounted on a graphite base so as to transform high-frequency radiation, X-ray radiation, into low-frequency radiation, in the visible spectrum. It is shown that if the CsI is grown on such a graphite base, a sensor with much better resolution and much better sensitivity is obtained than if a gadolinium oxysulphide scintillator were used. Precautions in preparing the graphite may furthermore be taken rendering the surface of the graphite denser. It can thus be covered with a layer of amorphous carbon and or be made to undergo impregnation. This densification contributes to the homogeneity of the sensor. Protection of the CsI is then effected by a synthetic resin layer allied with a layer of liquid resin for optical coupling with a detector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photosensitive matrix electronicsensor. It relates more particularly to X-ray radiation sensors. Theobject thereof is to improve the performance of sensors of this type.

2. Discussion of the Background

Radio logical image intensifier screens disposed opposite a detector andreceiving X-ray radiation on another face are known in the field ofX-ray electronic sensors. Scintillators are also known, in the field ofnuclear medicine, for transforming gamma rays (or X rays) into visibleradiation which can be detected by a detector. The detectors mostcommonly used are, in the field of radiology, cameras with target orassembled arrays of charge coupled devices (CCDs). In the field ofnuclear medicine, use is also made of banks of photomultiplier tubeslinked moreover to a barycentration electronic circuit. All thesesensors with detectors incapable of directly detecting X rays areassociated with scintillators tasked with transforming X rays intoradiation in the approximately visible spectrum.

The material used to perform the transformation, the material of thescintillator, is normally gadolinium oxysulphide. The latter is used inthe form of a thin deposit, typically of from 50 to 300 micrometers.This deposit consists of particles of this material which are boundtogether by a binder. The emission of visible light throughout thethickness, and in all directions, of this material causes a loss ofsensitivity and a loss of resolving power of the detector, and hence ofthe sensor.

A proposal has already been made to deposit a film (plastic) containinggadolinium oxysulphide on a luminous image matrix detector, the latterconsisting of a silicon integrated circuit.

Caesium iodide CsI, doped with thallium, in the form of needles, offersa beneficial alternative for greater luminous efficiency allied with awaveguide effect of the needles, whose cross sections have typicaldimensions of from 3 to 6 micrometers. This material is conventionallyused in radiological image intensifiers, by coating an input screenwhich generally consists of a domed aluminium foil. Embodiments are alsoknown in which a wad of optical fibres is covered by such a material.The needles are oriented perpendicularly to the surface of the supportwhich carries them. They only partially adjoin one another. They thusoffer a porosity of 20 to 25%. These air-filled pores, associated withthe favourable refractive index of CsI (1.78) afford channelling of thevisible photons emitted in each needle and impart a higher sensitivityand resolving power.

However, difficulties in using CsI, as compared with gadoliniumoxysulphide remain. One is aware, in particular, of the drawback thatCsI hydrates rapidly in ambient air and customary humidities. Thisuptake of water has the effect of degrading the image obtained with thesensor. It causes a halo effect initially. This humidification thenirreversibly degrades the needles with a consequent loss of luminousefficiency and resolving power of the sensor. It should be noted thatthis drawback is not encountered in radiological image intensifier tubessince the CsI is in the vacuum tube.

Moreover, although present in small quantities in the needles, thalliumis highly toxic. The low mechanical fastness of CsI then causes dust andwaste matter, the elimination of which must be scrupulously controlled.In certain cases, the passivation of the thallium-doped CsI is achievedby vaporizing a layer of aluminium on the surface of the scintillator.

On account of its low mechanical fastness, the CsI must be deposited ona rigid support. The bending of the support would in fact give rise tovisible defects in the image. This support must moreover normallyundergo, without deforming, a heat treatment to diffuse the thallium ata temperature of the order of 300°.

In radiological image intensifiers, the support is made of aluminium,sometimes allied with amorphous carbon, or even replaced with amorphouscarbon on account of the very great resistance of this material.

Outside of the construction of image intensifier tubes, the depositingof caesium iodide on beryllium has been envisaged. However, thismaterial has the drawback of being excessively expensive.

SUMMARY OF THE INVENTION

The object of the invention is to solve these problems by advocating thegrowing of a CsI layer on a base consisting of a machined graphiteblock, preferably having the particular feature of exhibiting lowsurface roughness. Preferably, in the invention the graphite used as abase has undergone, at its surface, a densification step so as toeliminate the natural porosity thereof related to the graphite.Moreover, this layer thus rendered denser is preferably then ground soas to impart a low roughness thereto. It has then been found that, whendeposited in the gaseous phase, the CsI adopts an entirely beneficialgrowth: the needles are regularly spaced and the surface of thescintillator thus produced is almost flat despite the defects related tothe roughness of the graphite.

If the base is not made to undergo the densification operation,differences in sensitivity result within the sensor produced. One canattempt to put this right. For example, if the graphite surface isstriated (for example with parallel lines), the presence of thesestriations is recognized in the image obtained after operation of thesensor. It is possible, especially in the field of nuclear medicine, tocorrect the differences in sensitivity relating to the various locationsthrough software processing. In an enhancement according to theinvention, one limits the magnitude of this correction through thedensification operation and/or the grinding operation.

In all cases, the presence of the graphite base affords the solution tothe problems of differential expansion occurring during the diffusion ofthe thallium.

Graphite, such as understood within the present invention, is a materialwhich differs from amorphous carbon in the sense that it has a porousphysical structure, unlike amorphous carbon which is very dense.Graphite can be machined with metal tools, whereas amorphous carbon isalmost only machinable with diamond-encrusted tools.

This is why, in the application envisaged here, namely the deposition ofcaesium iodide on a machined support intended to be placed in front of amatrix image detector, it proves to be especially beneficial to use agraphite block as support.

Graphite usually has a structure which is not only porous but alsolamellar, thereby further facilitating its machining, unlike amorphouscarbon, whose structure is essentially isotropic.

In principle graphite is obtained by compressing carbon powder at hightemperature, whereas amorphous carbon results from decomposition in thegaseous phase (cracking) culminating in the growth of coatings ofgreater or lesser thickness on a starting support. It is thereforeeasier to produce machinable blocks from graphite, whereas it appears tobe easier to produce amorphous carbon coatings on surfaces such as thedomed surfaces of radiological image intensifier input screens.

The subject of the invention is therefore a photosensitive matrixelectronic sensor comprising a matrix image detector surmounted by ascintillator for transforming high-frequency electromagnetic radiation,typically X-ray radiation, into low-frequency radiation, typicallyradiation in the visible domain, characterized in that the scintillatorcomprises a caesium iodide faceplate carried by a graphite base disposedon the side where the high-frequency radiation is received.

Its subject is also a process for fabricating a sensor, characterized inthat

a graphite base is made, this base having to serve as support for ascintillator,

the graphite base is ground,

caesium iodide is deposited in the gaseous phase on the graphite base,

the caesium iodide deposition is doped with thallium,

a layer made of a synthetic resin is deposited under vacuum in thegaseous phase on the caesium iodide deposition,

a layer of liquid optical coupling resin is deposited on the syntheticresin layer,

a detector is applied flat against the liquid optical coupling resinlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the description whichfollows and on examining the figures which accompany it. The latter aregiven merely by way of wholly non-limiting indication of the invention.The figures show:

FIG. 1: the diagrammatic representation of the structure of the sensoraccording to the invention;

FIG. 2: a diagrammatic representation of a machine used to implement aprocess for passivating the caesium iodide layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows, according to the invention, a photosensitive electronicsensor 1, preferably a matrix sensor. The sensor 1 comprises a detector2 surmounted by a scintillator 3. The purpose of this sensor is totransform X-ray radiation 4 or any other high-frequency radiation (whichcould also be gamma radiation) into low-frequency radiation 5. Theradiation 5 can thus be emitted within the visible spectrum. Theradiation 5 is then detectable by a detector 2. The detector 2 can be aconventional detector. In a preferred example, the detector 2 is of theCCD type, as indicated hereinabove. Each CCD device array forms a lineof detection points. Juxtaposed arrays serve to form the various linesof a matrix image.

The sensor essentially comprises a caesium iodide faceplate 6 carried bya graphite base 7. The base is disposed on the side where the X-rayradiation is received. The graphite used according to the invention ispreferably graphite with a lamellar structure, obtained byhot-compression of carbon powder. This type of graphite is inexpensiveto produce, and above all inexpensive to machine since it can bemachined with metal tools, whereas the structures of materials based onamorphous carbon can only be machined with diamond-encrusted tools.

The material used therefore takes the form of small agglomeratedlamellae 10, stacked end-to-end above one another. The thickness of thebase 7, in one example is of the order of 500 micrometers. In case thescintillator should be larger, it is possible to go up to 800 or 2000micrometers. It is possible to go down to 200 micrometers if it issmaller. In addition to its good permeability to X-rays, the graphiteaffords the advantage of being black, that is to say of absorbing thevisible radiations emitted in its direction by the scintillator andwhich contribute more to lowering the resolving power of the sensor thanto increasing its sensitivity. In a preferred example, the quality ofthe graphite of the base 7 will be such that the grain size, the lengthof the lamellae, will be less than 5 micrometers, preferably of theorder of or less than 1 micrometer. It has been observed in fact that,if the natural anisotropy of graphite were not controlled, it would leadto having grains of 20 micrometers. In this case, the quality ofregularity of the thickness of the CsI faceplate 6 was inferior, itrequired more software corrections.

The base 7 is preferably covered, so as to be surface-densified, with alayer of amorphous carbon 8. The layer of amorphous carbon 8, whosethickness is of order of 3 to 20 micrometers, makes it possible to fillin the holes 9 present on the surface of the base 7 owing to itsporosity. The atoms of the carbon layer 8 differ from those of the base7 in that, in the layer 7, the porosity is larger and thecarbon-graphite particles are oriented. The layer 8 of amorphous carbonis a denser layer which is unstructured, that is to say is notpolycrystalline: the atoms are aggregated there on top of one anotherwith no organization. This layer of amorphous carbon is for exampledeposited under vacuum in the vapour phase on the base 7.

As a variant, or as an adjunct, the graphite layer 7 may undergopreviously, at the location where the caesium iodide layer 6 will haveto grow, densification by impregnation. Such impregnation is for exampleachieved by covering that face of the graphite base 7 which is intendedto receive the caesium iodide with a film made of an organic resin. Thisassembly is then subjected to a very high temperature (1000°). Theeffect of this is to split the resin, to separate within the resin thecarbon atoms from the hydrogen atoms or other bodies to which they arebonded. These impurities are thus discharged naturally by evaporation.The effect of the high temperature is also to make the carbon atomsmigrate by diffusion into the porosity spaces 11 of the base 7. Todensify the useful surface of the base 7 still further, thisimpregnation operation can be repeated several times so as to increasethe compact nature. In one example it is performed four times running.

As was indicated above, one may decide to allow imperfections in thesurface to exist. In this case, one accepts that the essential aspect ofthe correction of the images obtained can be postponed to a softwareprocessing subsequent to their being obtained. In the invention, agrinding of the graphite surface is preferably effected, in particularafter densification, with a grinding tool 21. Typically, the grindingaction removes a small thickness of 5 to 10 micrometers from the toplayer of the base 7 or from the layer 8 as appropriate. The depositingof the layer 8 can take place before or after grinding. This results ina roughness h of the order of from 0.2 to 0.4 micrometers, whereaswithout grinding the natural roughness H, especially withoutdensification, may reach 130 micrometers, in particular if the size ofthe grains of the graphite is of the order of 20 micrometers.

The CsI is then grown by acting in a conventional manner. Needles 12 arethus obtained, the dimension of whose cross section is of the order offrom 3 to 6 micrometers in diameter. The cross sections of the needles12 can be of various sizes as is apparent in FIG. 1. In one example, theneedles 12 are separated from one another, randomly, by a space 13 ofbetween 1 to 3 micrometers. This space makes it possible to construct,with the needles 12, a change-of-medium face 14. The presence of thisface 14 allied with the favourable refractive index of the CsI leads tofibre-optic-like operation of the needles 12. Stated otherwise, thetransformations of radiation, the scintillations which occur in a needle12, give rise to a radiation 5 which will be guided. If this radiationis oriented on emission thereof towards the detector 2, it exitsnormally from the needles 12 through their apex 15. On the other hand,if this radiation 5 is oblique, it is reflected, inside the needles 12off the face 14 and finally exits through the apex 15. The portionemitted towards the base is absorbed by the black base 7. In oneexample, the layer 6 of CsI has a thickness of between 100 to 300micrometers. Typically, it measures 180 micrometers.

The layer 6 is then itself doped with thallium in a conventional manner.

Lastly, the layer 6 of doped CsI is covered with a passivation layer 16.As compared with the prior art in which the passivation layer 16 was asilicone gel, involving gadolinium oxysulphide, the invention advocatesthat the passivation layer 16 be produced in the form of a transparentpolymerized synthetic resin. This polymerized resin having the advantageof being more impervious and of preventing the evaporation of dust fromCsI or from thallium, has the drawback however of not leading to aperfectly smooth outer surface. In the invention, the passivation layer16 is then allied with a layer 17 of liquid resin for optical couplingwith the detector 2. In this way, good thallium evaporationimperviousness is obtained without impairing the efficiency of thesensor.

FIG. 2 shows a machine which can be used to produce the passivationlayer 16. This machine comprises three cells linked together. In a firstcell 18, the material for producing the resin is introduced raw. In apreferred example this material is di-paraxylylene. This material isvaporized in the cell 18 at a temperature of 175° under a pressure ofone torr (one millimetre of mercury). The first cell 18 is connectedwith a second cell 19 in which the vaporized material is subjected tovapour deposition. For example, the di-paraxylylene vapour is heated to680° under a pressure of 0.5 torr. Subjected to this stress, thedi-paraxylylene splits and is transformed into monomer paraxylylene. Theparaxylylene thus prepared is introduced at ambient temperature andunder a very low pressure of 0.1 torr into a third cell 20 where it isdiffused as layer 16 over the needles 12 of the layer 6. Theparaxylylene then recombines to form a polyparaxylylene polymer bycondensation. This condensation leads to the production of bridges abovethe porosity spaces 13 of the CsI layer without penetrating into thegaps.

It is possible to use a synthetic resin other than the resin designatedhereinabove. The latter has the advantage however that it adheres wellto CsI on the one hand and, on the other hand, that it allows theconstruction of bridges above the spaces 13 without filling in thesespaces. Preferably, the resin used shall have a refractive index ofbetween 1.78 and 1.45. Therefore, this resin having an index below thatof CsI forms, on bonding with the latter, an antireflection layer. Inone example, the layer 16 has a thickness of from 1 to 25 micrometers.

The liquid resin layer 17 is then spread over the passivation layer 16(and remains there) so as to ensure good optical coupling. This resinpreferably has a refractive index of less than 1.45. It is for exampleof the type of those used in the construction of liquid crystal cells.The thickness of the layer 17 is of the same order as that of the layer16.

The detector 2 is then fixed to the base 7 by conventional mechanicalmeans.

What is claimed is:
 1. A photosensitive matrix electronic sensorcomprising: a scintillator having a caesium iodide faceplate depositedon a graphite base; and a matrix image detector surmounted by thescintillator for transforming high-frequency electromagnetic radiationinto low-frequency radiation, wherein the caesium iodide faceplate isdisposed between said matrix image detector and said graphite base. 2.The sensor according to claim 1, wherein the base of the scintillatorincludes graphite whose grain size is less than 5 micrometers.
 3. Thesensor according to claim 1, wherein the base includes graphite coveredwith a layer of amorphous carbon.
 4. The sensor according to claim 1,wherein the base includes graphite impregnated with carbon.
 5. Thesensor according to claim 1, wherein the faceplate of the scintillatoris insulated from an ambient medium by a passivation layer including asynthetic resin covered with an optical coupling layer.
 6. The sensoraccording to claim 5, wherein a refractive index of the resin of thepassivation layer lies between 1.78 and 1.45 so as to form anantireflection layer.
 7. The sensor according to claim 5, wherein thegraphite base has a thickness of 200 to 2000 micrometers, the caesiumiodide faceplate has a thickness of 100 to 300 micrometers, and theresin passivation layer has a thickness of 1 to 25 micrometers.
 8. Aprocess for fabricating a matrix image sensor, comprising: making agraphite base to serve as support for a scintillator; grinding thegraphite base; depositing caesium iodide in a gaseous phase on thegraphite base; doping the caesium iodide deposition with thallium;depositing a layer made of a synthetic resin under vacuum in the gaseousphase on the caesium iodide deposition; depositing a layer of an opticalcoupling resin on the synthetic resin layer; and applying a detectorflat against the optical coupling resin layer.
 9. The process accordingto claim 8, further comprising: treating a surface of the graphitebefore or after grinding by depositing a layer of amorphous carbon. 10.The process according to claim 8, further comprising: treating a surfaceof the graphite before grinding by impregnation.
 11. The processaccording to claim 9, further comprising: treating a surface: of thegraphite before grinding by impregnation.
 12. The sensor according toclaim 1, wherein the high-frequency electromagnetic radiation is x-rayradiation and the low-frequency radiation is visible radiation.
 13. Thesensor according to claim 1, wherein the base of the scintillatorincludes graphite whose grain size is less than or equal to 1 μm. 14.The sensor according to claim 5, wherein the synthetic resin is apolyparaxylylene resin.
 15. The sensor according to claim 5, wherein thegraphite base has a thickness of 500-800 μm, the caesium iodidefaceplate has a thickness of 180 μm, and the resin passivation layer hasa thickness of 1 to 25 μm.